The Mercury-bound MESSENGER spacecraft completed its fifth and final deep-space maneuver of the mission today, providing the expected velocity change needed to place the spacecraft on course to enter into orbit about Mercury in March 2011. . . . today's maneuver began at 4:45 p.m. EST. Mission controllers at The Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md., verified the start of the maneuver about 12 minutes, 49 seconds later, when the first signals indicating spacecraft thruster activity reached NASA's Deep Space Network tracking station outside Goldstone, Calif.

"The team was well-prepared for the maneuver," said MESSENGER Mission Systems Engineer Eric Finnegan, of APL. "Initial data analysis indicates an extremely accurate maneuver execution. After sifting through all the post-burn data I expect we will find ourselves right on target."

"A 3.3-minute firing of its bi-propellant engine provided nearly all of the probe’s 177 meter per second (396 mile per hour) increase in its speed relative to the Sun."

Quite a little kick -- it moves the periapsis of Messenger's orbit very close to Mercury's. Somehow, capture at periapsis is most energy-favorable, but it is not casually obvious why. (Maybe someone will be kind enough to post an explanation...)

I think the Oberth effect is only an oblique influence in the positioning of the capture point of Messenger in Mercury's orbit. There may be some advantages from the positioning of the 3-body problem as far as Messenger's entry into Mercury's gravity well, but there were not rocket firings in any of the fly-by's, and the capture rocket firing will be for losing energy relative to Mercury (admittedly centered at Mercury closest approach, so that aspect of the capture rocket firing is Oberth-enhanced). What is not obvious is why this near-Mercury capture is more efficient at Mercury's perihelion, rather than elsewhere in its orbit.

Perhaps the net trajectory deflection by Mercury's gravitational field can be viewed in some sort of approximation as a delta-v impulse, and that is why the fly-by's and capture are grouped near Mercury's perihelion, and that is how the Oberth effect is realized.

That was quick! This is from Jim McAdams, MESSENGER Mission Design Lead Engineer. I can't say I understand it all -- the physics of trajectories is not one of my strengths.

QUOTE (Jim McAdams)

The lowest arrival velocity for a spacecraft on a ballistic trajectory approaching Mercury is achieved when Mercury is at perihelion. For much of the interplanetary cruise phase, this option served as a contingency back-up for the MESSENGER prime trajectory. As much as 200 m/s delta-V savings were possible with this option. This option required an additional two Mercury flybys (one a few days before the current MOI and another 88 days later) and an increase in launch-to-MOI time of 128-130 days – slightly less than 1.5 orbits of Mercury around the Sun.

Now here is why this trajectory option was not chosen as the baseline for MESSENGER.

The timing of MESSENGER propulsive maneuvers is based on selecting a spacecraft orientation that positions the sunshade between the Sun and sensitive components of the spacecraft. At close range to the Sun, permanent damage or spacecraft failure can occur in as little as an hour without sunshade protection. A peer-reviewed complex orbit insertion sequence of 6-7 maneuvers (vs. 1 or 2 maneuvers for the nominal mission plan) is required to place the spacecraft in the science-defined initial orbit about MESSENGER. This process takes over 6 weeks, adding significant risk and further delay. The primary reason that the sequence is so complex is that the Mercury-relative arrival geometry leads to an initial orbit inclination of 89-90 degrees – not the 80-83 degrees desired for initial orbit inclination. To achieve a low-cost orbit inclination change, the initial orbit must have a much larger orbit period. This subjects the orbit to large perturbations from solar gravity. Solar gravity alone can be used to make most or all of the orbit inclination change from 89-90 degrees to 80-83 degrees, but other changes to the orbit introduce the need for multiple corrective maneuvers after initial orbit insertion. With arrival near Mercury’s aphelion and achieving the required spacecraft-Sun-relative orbit orientation for thermal stability, the solar gravity perturbations have the OPPOSITE effect that they do for the primary mission with orbit insertion near Mercury’s perihelion. That is, periherm altitude decreases, leading rapidly to impact with Mercury’s surface in the absence of corrective maneuvers. So the orbit periherm altitude no longer drifts from 200 to 450-500 km followed by periherm-lowering delta-V, but drifts from 450-500 km down to 200 km with periherm-raising delta-Vs. After onboard propellant runs out, the potential for extended mission options would be very minimal. Another complicating factor is that the 5th Mercury flyby must occur at relatively low altitude with the spacecraft flying between the Sun and Mercury when Mercury is near its perihelion – subjecting the spacecraft to heat from solar radiation off Mercury’s surface. This altitude can be kept sufficiently high, but there still is a substantial increase of thermal input to the sensitive portions of the spacecraft.

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I think it's more efficient at Mercury''s perhelion simply because the relative velocities between messenger and Mercury are smaller. MEssenger is moving closer to the Sun as it goes from Earth to Mercury. So, it will be in an orbit where it meets Merury close to its perihelion. At this point it will be moving faster than Mercury (it has a larger orbit for the same position, so the velocity at that place must be larger). At Mercury's perihelion it will be moving fastest, so the velocity difference between Messenger and Mercury will be smaller. It is slightly closer to the Sun, so also Messenger will be moving a bit faster, but relatively less so than the increase in Mercury's speed, because (sorry for the maths):

A quick read tells me the perihelion option would save propellant during the initial insertion, however the inclination wouldn't have been too favorable, and more flybys would have been needed, so they chose another option. By the way I like the term "periherm".

That was a great reply and discussion about capture at perihelion. After thinking about it a bit, an easier-to-understand visualization came to mind...

When Mercury is at perihelion, the sun's gravity is stronger relative to Mercury's at a given distance from Mercury (compared to Aphelion), thus reducing the size of Mercury's "gravity well", and thus requiring less delta-v to stay in it.

So Messenger can get closer to Mercury before the dynamics become Mercury-dominated, rather than Solar-dominated. (Remember, this is a visualization, not a precision calculation...)

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